A lot of tools already convert $GALAXY_MEMORY_MB to GB - there is no reason we shouldn't also just automatically set this in addition to the MB version.

Additionally, should we set both MB and GB lower than the actual limit? Or offer some var that is less than the actual limit? Generally if there is a DRM-enforced hard limit for the job and the tool can actually limit its memory usage, it has to be set a GB or two lower to actually avoid hitting the limit.